The invention relates to an objective adjusting device having a stationary housing, an adjusting ring with an internal thread and a tube, with an external thread, that is guided rectilinearly in an axial direction and carries optical active elements, the adjusting ring being rotatably supported in the housing, and the tube having at least one axially parallel opening with mutually parallel sliding surfaces in which a guide element connected to the housing engages.
Such objective adjusting devices are used for focusing photographic objectives. It is known for this purpose to displace the focusing element in an axial direction along the optical axis of the objective. During the displacement, rotations of the lens elements are preferably to be avoided in order to exclude influences of centering errors which may be present. The rotary movement of a focusing ring is converted into a precise axial movement via a rectilinear guide running parallel to the optical axis and secured in terms of rotation. The axial movement of the focusing optics must be free from radial play in the event of strict requirements placed on optical accuracy. For this reason, but also in order to be able to implement large leads and thus large axial travel, the thread is designed as a helical thread. The rectilinear guide must run in the exactly axially parallel fashion in order to ensure that no overdetermination, which would result in a sluggishness of the axial movement, comes about in conjunction with the positively driven tube of this helical thread. The freedom from play of the guide element must likewise be ensured.
DE 198 04 472 C1 discloses a play-free rectilinear guide in a cylindrical mount in which the stationary guide track running parallel to the optical axis has on one side a stop strip, resiliently supported by a wave spring, as a running surface for the cylindrical guide element. Because of the pressure acting through the wave spring only on one side, the guide element, which is designed in a solid fashion as a cylindrical pin which can be screwed in, runs along the guide track under a permanent load on one side during the axial displacement movement. In the course of time, the biasing acting on the cylindrical pin varies owing to wear of the guide track and to fatigue of the wave spring, something which has a negative effect on the freedom from play.
DD 274 287 A1 discloses a rectilinear guide which comprises a bent guide part which engages in a guide groove running parallel to the optical axis, and is anchored on the body of a mount by two screws. In the case of this rectilinear guide, the guide part must be precisely fitted in the region of its axially parallel extent to the width of the guide groove, in order to avoid inverting images and inaccuracies in focusing. Very stringent requirements are to be placed on the parallelism of the guide part relative to the guide groove and to the optical axis, in order to avoid torsional stresses and uneven running of the tube supporting the optical active elements over the axial travel provided.